Warpage-reducing packaging design

ABSTRACT

A flange and packaging assembly characterized by a structural lip configured to impart resistance to thermal-induced deformation (i.e. “stiffness”) to the flange during elevated temperature die attachment processes. The stiffness is achieved through appropriate configuration of the lip (height, width and shape) and is sufficient to at least partially counteract the stresses generated from coefficient of thermal expansion differentials between the flange and a die attached to it. The lip may be molded or otherwise formed with the flange as a unitary lip-stiffened flange, or may be a separate structure that is affixed to the flange. The flange has a lip-stiffened region for die attachment. This region may be raised above the first surface of the substrate base for enhanced stiffening, and may be in the form of a single pedestal supporting a single or multiple die, or may be a series of raised pedestals, each supporting one or more die. The leads may be embedded in a polymetric material that is attached to the flange that is of sufficient thickess to provide the required stiffness. The embedded leads may be shaped and formed so that the desired seating plane height is achieved. The lead form placement can be either on the interior or exterior of the embedded polymetric material. The polymetric material can be supported along the edge of the flange or be seated on a part of an extrusion from the flange.

TECHNICAL FIELD

The present invention relates generally to semiconductor devicefabrication. More particularly, the present invention relates tosemiconductor packaging for reduction of thermal-mechanical deformationof the flange known as “warpage.”

BACKGROUND

It is well known that after semiconductor dies are fabricated on asemiconductor wafer, such as but not limited to, silicon or galliumarsenide, the wafer is sawn (also referred to as “diced”) to separatethe individual semiconductor die (also known as “dice” or “chips” or“device”) on the wafer. These separated die are then each packaged tofacilitate safe die handling, and attachment to circuit boards, heatsinks, and the like. During this process, the die is attached or mountedto a packaging substrate (also known as a “flange”). In the case of somedevices, such as power amplifiers, where heat generation is expectedduring operation of the device, the die with packaging substrate backingis then mounted or attached to a heat sink that acts to remove generatedheat by conduction away from the device in use.

In a typical packaging process, the die is attached to the flange by anysuitable method, for example by soldering or using an adhesive. Thisprocess is carried out at elevated temperature, and the die-flangecombination is subsequently cooled to room temperature.

For conventional materials used in packaging, such as those described inU.S. patent publication 20050016750 A1, there is a tendency for theflange to deform from planarity or “warp,” upon cooling after the dieattachment process. The deformation leads to further manufacturingproblems in subsequent processes when the window frame, leads and lidare attached. It also to leads to inefficient transfer of heat generatedby the die, while it is in use, to the flange and heat sink.Consequently, the warpage prevents the maximum potential heat transferfrom die to flange and from flange to heat sink. This leads totemperature increase at the die, with undesirable consequences.

In another configuration, U.S. patent publication 20050012118 describesa package that can withstand high die-attach temperatures and that canprovide a hermetically sealed air cavity for a die, without the use ofadhesives. It discusses a circuit package for housing semiconductor thathas a metallic flange, one or more high-copper leads and a liquidcrystal polymer frame molded to the flange and the leads. The flangeincludes a dovetail-shaped groove that mechanically interlocks with themolded frame. During molding, a portion of the frame forms a key thatfreezes in or around the frame retention feature. There is no discussionof heat-induced flange deformation.

The art recognizes that heat transfer from die to flange poses achallenge in operations where the die is attached at high temperature,and the combination is subsequently cooled to room temperature andthermal-mechanical deformation is induced.

Accordingly, it is desirable to minimize the warpage of the flange inthe die attachment process. In addition, it is desirable to develop asolution that retains the highest possible heat transfer from die toflange, and from flange to heat sink. Furthermore, other desirablefeatures and characteristics of the present invention will becomeapparent from the subsequent detailed description and the appendedclaims, taken in conjunction with the accompanying drawings and theforegoing technical field and background.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, which are not represented asbeing to scale, wherein like reference numbers refer to similar elementsthroughout the figures.

FIG. 1 is a schematic depiction of half of a prior art air cavitypackage;

FIG. 2 is a schematic depiction of half of another prior art air cavitypackage with a bonded ceramic insulator frame;

FIG. 3 is a schematic depiction of the half of the prior art assemblyproduct of FIG. 2 with attached leads;

FIG. 4 is a schematic perspective view of half of an embodiment of theinvention showing a flange with stiffening lip and die on a diepedestal;

FIG. 5 is a schematic perspective view of half of an embodiment of theinvention showing a raised stiffening lip and multiple die, eachdisposed on its own pedestal;

FIG. 6 is a schematic perspective view of half of an embodiment of theinvention with embedded leads extending through a molded, stiffeninglip;

FIG. 7 is a perspective view of half of an embodiment of the inventionwith embedded leads configured to maintain a standard seating plane anddie on multiple pedestals;

FIG. 8 is a top view of the embodiment of the invention showing athicker flange with embedded leads, extending through a lip in aperipheral region of the frame, while seating plane s is maintained;

FIG. 8 is a schematic cross sectional view of the embodiment of FIG. 8with a single pedestal;

FIG. 10 is an alternative embodiment to FIG. 9 with multiple pedestals;and

FIG. 11 is a graphical representation of the improvement in flangestiffness over a control achieved in accordance with the invention, forparticular lip and pedestal configurations and under specifiedconditions.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the invention or the application and uses ofthe invention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, or the following detailed description. In addition, for thesake of brevity, conventional techniques related to finite elementanalysis, finite difference analysis, and thermal modeling may not bedescribed in detail herein

In a package designed to remove heat from an operating device, like apower amplifier, the main mechanism for heat transfer from the die tothe package substrate (also known as the “flange”) to which it isattached directly, is by conduction. Heat transfer by conduction isdependent, among other parameters, upon the surface area of the die thatis in direct contact with the flange. All other factors being equal,heat transfer increases as contact area increases. Accordingly, completephysical contact over the entire interfacial area between die and flangemaximizes heat transfer by conduction.

A predominant cause of loss of heat transfer efficiency from a die tothe flange is physical deformation or “warpage” of the flange leading toseparation between the die and the flange along their interface. Thewarpage often arises during the die attachment process, which is carriedout at elevated temperatures. For some packaging materials used instandard packaging the warpage can be in excess of 5 mil. The extent ofwarpage varies, based on several factors (die attachment temperature,flange thickness, number of die attached, semiconductor and packagingmaterial properties, etc.) but can be significant. Excessive warpage hasundesirable effects in fabrication as well as subsequent use of thedevice. Thus, for example, warpage must be minimized to avoiddeleterious effects on subsequent assembly operations that must beperformed on the package. In addition, the physical separation betweendie and flange results in a gap that, whether air-filled or not,disrupts heat transfer by conduction from die to flange (which thentransfer heat to the heat sink). The gap reduces the area of the die indirect contact with the flange and thereby reduces conductive heattransfer.

Heat transfer rates from die to flange can be maintained by preventingor minimizing the extent of flange warpage that causes separation duringdie attachment processes. Warpage can be reduced to some extent byappropriate selection of materials. For example, flange materials may beselected from those that have a coefficient of thermal expansion (“CTE”)that closely approximates or more closely matches the CTE of the die.This selection will avoid or minimize the build up of thermally-inducedmechanical stresses at the die-flange interface that may cause physicalseparation of die and flange along at least a part of the interface.However, those materials that have a CTE that closely approximates theCTE of the die tend to have poor thermal conductivity (“TC”) properties.A high TC is essential for high rates of heat transfer from die toflange and then through the flange to the heat sink. This means that aflange material with a poor TC may allow the die temperature to rise(because of inability to conduct heat away) and the temperature mayreach a point that could result in adverse performance, diminishedreliability, or even die damage.

Accordingly, the invention provides modified flanges that have physicalfeatures to reduce the tendency of the flange to warp under thermallyinduced stresses, and that reduces the tendency for separation at theinterface between the flange and the die.

More particularly, in one embodiment, the invention provides a packagingsubstrate base (also known as a “flange”) that has a structural lipconfigured to impart “stiffness” to the substrate base. The term“stiffness” here means the degree of resistance to thermally-inducedwarpage, where the warpage results from attachment of die to a flangethat has a different coefficient of thermal expansion from the die.

The flange-stiffening lip may be made from a variety of materials butcommonly would be the same as the flange body. The flange may be made ofa conductive material such as copper-tungsten, copper-molybdenum,copper, and high conductivity ceramics. The stiffness of the raised lipis achieved through appropriate configuration of the lip (height, widthand shape) and is sufficient to at least partially counteract thewarpage-inducing stresses generated from coefficient of thermalexpansion differentials between the flange and a die attached to it. Thelip may be molded or otherwise formed with the flange as a unitarylip-stiffened flange, or may be a separate structure that is affixed tothe flange. In general, the packaging flange has a first surface thathas a first region having the stiffening lip. The first surface also hasa second region for die attachment. The second region is a region thatis stiffened by the lip, and may be raised above the first surface ofthe flange. The second region may be in the form of a single pedestalsupporting a single or multiple die, or may be a series of raisedpedestals, each supporting one or more die.

In another embodiment, the invention provides an assembly of die on thelip-stiffened flange. In addition to the flange, the assembly alsoincludes at least one die in the second region of the flange. Tofacilitate electrical communication, the assembly includes leads forelectrical contact with the die. The leads and flange are insulated fromeach other by means of an intermediate material, such as a(non-conductive) ceramic layer, interposed between the flange and theleads. In one embodiment, the leads are configured to preserve a seatingplane at the desired height and configuration. The leads may be embeddedin an organic composition. Further, the leads may have an end-portionembedded in and extending along a length of the lip; or the lead endportions may be attached along an upper surface of a length of the lip.

The stiffening lip may take any of a variety of suitable shapes. In oneembodiment, the lip is a raised wall that extends around a peripheralarea of the flange. Taking into account that the flange is laterallydivided in the accompanying drawings, the lip is shown U-shaped, butshould be understood to include its mirror image to form a frame-shape.Other raised configurations that also provide resistance to heat-inducedstress deformation of the flange are, of course, also useful andcontemplated in the invention. For example, the lip may be in the shapeof a rectangular array of a pair of spaced apart longitudinal and a pairof spaced apart lateral rails, where the rails are not connected up toform a frame. Or the stiffening features could be a series oflongitudinally extending rails, such as two or three or more parallelrails.

It is useful to consider the prior art before turning to details ofembodiments of the invention depicted in the accompanying figures. Inreferring to the attached figures showing flanges of the invention andprior art flanges, it should be noted that the flanges are symmetricalabout a lateral axis at their mid points. Each depicted flange has beencut in half laterally at the midpoint, so that only one half is shown,for simplicity.

Referring now to FIG. 1, which is half of a symmetrical device obtainedby cutting it laterally across a plane of symmetry, a flange 10 is aplanar substantially rectangular block of appropriate conductivematerial such as copper, to which are attached die 12, which is but notlimited to semiconductor device like silicon. In this particular casethere are six die arrayed in two rows of three each (i.e. twelve in theentire package). Note that upper surface 14 of the flange 10 is planarand devoid of raised features.

Referring further to prior art depicted in FIG. 2, also a half of asymmetrical device, the flange 20 is similar to that of FIG. 1, but hasan adhered non-conductive ceramic frame 22 extending at the peripheryand around a central area of the flange where the die 24 are attached.

Referring to FIG. 3, the prior art of FIG. 2 is depicted with leads 26attached to the upper surface 23 of the ceramic frame 22. The leadsextend outboard away from the flange at a height and in a plane thatdefines the “seating plane” for the leads to facilitate subsequentprocesses involving the leads.

Turning now to embodiments of the invention, FIG. 4 is half of asymmetrical device, cut along a lateral plane of symmetry. FIG. 4depicts half a flange 100 with an upper surface 102 and an under surface(not shown) 104. A stiffening lip 200 extends along a periphery of theupper surface 102. In this particular embodiment, the lip has a U-shapewhen halved laterally (i.e. in full shape it is a frame), although othershapes may also be useful and are within the scope of the invention.Also, the lip has a width w and a thickness or height h above surface102. The height h and width w are selected to enhance stiffness. Thesedimensions will vary depending upon the size (thickness, length) of theflange and type of flange material, for example. In general however, agreater height h provides more stiffness than a lower height, ingeneral, all other factors being equal. Similarly, a greater width wprovides more stiffness to the flange than a lesser width, in general,all other factors being equal. In general, the height h of the lip wouldbe in the range from about 10 to 25 mil for a copper flange. The heighth of the lip will vary with materials and geometry of the flange anddie, and materials with smaller CTE differences from the die shouldrequire smaller h. Smaller h would also be expected for comparativelythicker flanges or when using comparatively thin die, all other factorsbeing equal.

The stiffening lip may be molded with the flange as a one piece unit, ormay be subsequently attached to the flange. The stiffening lip may be ofthe same or different material from the flange. The lip is preferably ofthe same material as the flange, or of a material with similar CTE. Thelocation and size of the area that is stiffened by the lip also dependsupon the configuration and location of the lip. The die may be attachedin the stiffened area for best effect.

In FIG. 4, a pedestal 300 is disposed in the lip-stiffened area of theflange 100, and is shown in the form of a raised platform. The raisedpedestal 300 provides some enhancement of flange stiffness as well, asshown in the Example provided here below. Die (six die 400 are shown inthis example, twelve in the symmetrical device) are attached to thepedestal to take advantage of the stiffened structure in that regionthat is at least partially surrounded by the stiffening lip 200.

FIG. 5 depicts another embodiment of the invention, this one also shownin symmetrical half view like that of FIG. 4. However, in this case thepedestal on flange 100 is not a single pedestal but a plurality ofraised pedestals 330, each having an upper surface adapted for dieattachment. In the embodiment shown the pedestals 330 are sized toconform to and register with the size of the die, however, the pedestalsmay each be oversized relative to the die. In addition, more than onedie may be attached to each pedestal 330, depending upon selected dieand pedestal sizes.

FIG. 6 is a schematic depiction, according to an embodiment of theinvention, of half of a symmetrical product assembly. In thisembodiment, the half of the raised lip 200 is of a U-shape (the completelip is a frame) and disposed to surround die 400 that are disposed onthe upper surface 102 of flange 100. The leads 500 have inboard endportions 502 that are embedded into and extend along the length of thewall 210 of the lip 200. Lead end portions 502 are isolated from flange100 by surrounding non-conductive materials, such as ceramic layers 540,shown. Further the leads 500 are configured so that when end portions502 are embedded into the lip 200, lead outboard end portions 504 are ata height s (the seating plane) from a plane that is co-extensive withflange under surface 104. Accordingly, very few if any toolingmodifications are necessary to accommodate the design for subsequentprocessing.

FIG. 7 depicts another (lateral symmetrical half of) embodiment of aproduct assembly of the invention. Here, flange 100 has a stiffening lip200 and die 400 are each attached to one of a plurality of pedestals330. Of course, a single raised pedestal with one or a plurality of dieattached may also be used. The leads 500 are shaped such that wheninboard end portions 502 are attached to the assembly, outboard endportions 504 are in the seating plane s. The inboard end portions 502 ofleads 500 are embedded in the proximity of the upper surface of the lip200 on ceramic inter-layer 540. Alternatively, the lead end portions maybe embedded into and extend along the lip wall itself 510, whileensuring electrical isolation from the flange. The leads 500 are shapedso that outboard end portions 504 are at an elevation below the inboardend portions 502 when the leads are attached. End portions 504 will liein a plane that is a height s (the desired seating plane) above flangeundersurface 104, as also shown in FIG. 6.

FIGS. 8, 9, and 10 represent top and alternative cross sectional views,respectively, of embodiments of the invention. Referring first to FIG.8, a common top view, and FIG. 9, the flange 100 has a raised lip (shownhere as a pair of walls 200 extending longitudinally on each side of theflange 100) that stiffens the area between the lip walls 200. Aplurality of die 400 is attached in the stiffened area, on a raisedplatform 300, as seen from FIG. 9. The need for raised platform 300 isdiscretionary since the flange thickness t could be increased to provideidentical stiffness as provided by the coupled system of flange 100 andraised platform 300. From FIGS. 8 and 9, inboard end portions 502 ofleads 500 are embedded in opposing lip wall sides 210, and extend upwardthrough the lip walls 210 to be exposed on the upper surface of the lipwalls for attachment of connecting leads 550 to die 400. Walls 200 maybe of a nonconductive material, such as ceramic or a polymeric material,to ensure isolation of leads 500 from flange 100. In another embodiment,the leads may be isolated electrically from the lip walls 200 by acoating or intermediate layer of a non conductive of material. Leads 500are configured so that outboard end portions 504 are in the desiredseating plane s. Clearly, the seating plane s can be adjusted bycontrolling the shape of leads 500 taking into account the shape of thedevice and the seating plane necessary.

As shown in FIG. 9, the flange 100 is of greater thickness t than priorart flanges. The thickness t is sufficient to provide resistance tothermally-induced warpage, and is typically in the range from about 65to 100 mil. The flange 100 has a pair of raised walls 200 that extendaround the sides of the flange 100 to form outer side walls 210. Theraised walls 200 are penetrated by lead end portions 502 that extendthrough the walls 200 (all along the length of the walls) to be exposed,for electrical connection, above walls 200 to lead wires 550. These leadwires 550 also connect to the die 400 that are all attached to a singleraised pedestal 300. Clearly, each die 400 might also be attached to oneof a multiplicity of raised pedestals 330, as shown in FIG. 10. The needfor raised pedestal 330 is discretionary since the flange thickness tcould be increased to provide identical stiffness as provided by thecoupled system of flange 100 and raised platform 330.

The embodiment of FIG. 10 is similar to the embodiment of FIG. 9 exceptthat the raised walls 200 rest upon a platform 120 on the perimeter ofthe flange 100. Thus, the sides of the device are made up of: (1) uppersidewalls that are the sidewalls 210 of the walls 200; and (2) and lowersidewalls that are the flange sidewalls 110, 112. Leads penetrate thewalls 200 as in FIG. 9. Above comments, with reference to FIG. 9,regarding lead electrical isolation from the flange apply here as well.

In general, in one aspect, the invention provides a packaging flangethat has a first surface and a second surface opposed to the first. Thefirst surface of the flange includes a first region that has a raisedlip extending thereon. The first region is spaced from a second regionthat is adapted to receive at least one die. The lip is configured toimpart stiffness to the flange, the stiffness sufficient to at leastpartially counteract thermally-induced stresses generated fromcoefficient of thermal expansion differentials between the flange and adie during an elevated temperature die attachment process. The secondregion may include a pedestal that has a surface sized and shaped tosupport at least one die thereon. Or, the second region may include aplurality of pedestals. The lip may extend around a peripheral region ofthe first surface of the flange to form a frame surrounding the secondregion. The flange and the lip may be molded in one piece. The ratio ofstiffening lip height h to flange thickness t for a configuration likethat shown in FIG. 8, using copper flanges, would be in the range fromabout 0.15 to about 0.25 for a 100 mil thickness flange. The ratio willvary based on several considerations, including flange configuration,materials, and the like.

In another aspect, the invention also provides a die assembly that has aflange having a first surface and a second surface opposed to the first.The first surface of the flange includes a first region having a lipextending thereon. The first region is spaced from a second region thatis adapted to receive at least one die. The lip is configured to impartstiffness to the flange, the stiffness sufficient to at least partiallycounteract thermally-induced stresses generated from coefficient ofthermal expansion differentials between the substrate and a die to bemounted thereon in an elevated temperature die attachment process. Theflange has at least one die mounted in the second region. The assemblyincludes leads for electrical contact with the die. The leads may beconfigured to preserve a seating plane, which is spaced from a plane ofthe second surface of the flange. In addition, the leads may be embeddedin an organic composition, with inboard end-portions embedded in thelip. The leads may be embedded in an organic composition so that themolded leads extend from raised inboard end portions nearest the die tolower outboard end portions that are aligned substantially within theseating plane. The inboard end portions of the leads may be exposed forelectrical communication with the die. Further, the second region mayinclude a pedestal having a surface adapted to support at least one diethereon. Or, the second region may have a plurality of pedestals. Eachof the pedestals has a surface that may be adapted to support at leastone die thereon.

In yet another aspect, the die assembly includes: a packaging flangehaving a first surface and a second surface opposed to the first, thefirst surface of the flange comprising a first region having a lipextending thereon, the first region spaced from a second region adaptedto receive at least one die, the flange having a body configured toimpart stiffness to the flange, the stiffness sufficient to at leastpartially counteract thermally-induced stresses generated fromcoefficient of thermal expansion differentials between the flange and adie to be attached thereon during an elevated temperature die attachmentprocess. The assembly of claim 14, wherein the body of the flange hasthickness in the range from about 65 to about 100 mils. The first regionmay extend along a side of the flange and the lip may extend upward as awall above the upper surface of the flange. The first region mayalternatively extend along a side of the flange, where the flange has aplatform so that the lip rests on the platform. There may be at leastone pedestal in the second region, or none. The assembly may have leadswith inboard end portions embedded along a length of a lip wall andextending to an upper surface of the lip to permit attachment ofconnections to die on the flange.

The following example and test results are intended to illustrate someof the many improvements that the invention provides, but does not limiteither the scope of the invention, or the benefits thereof.

EXAMPLE

Tests were conducted to quantify the improvement in stiffness(resistance to warpage from heat-induced stress) of flanges madeaccording to the invention as compared to a control without anystiffening structure. The flanges were all made of copper and 100 milthick. In these tests the following were compared:

A. A 100 mil thick flange as control

B. A 100 mil thick flange with a 21 mil height frame

C. A 100 mil thick flange with one 21 mil raised pedestal (no frame)

D. A 100 mil thick flange with 21 mil height frame and one 21 mil thickpedestal

E. A 100 mil thick flange with 21 mil height frame and six 21 mil thickdie stands

Each flange was measured for planarity to eliminate any that hadinherent warpage. Testing was then conducted according to the following:the flanges were each heated to 380 degrees Centigrade for dieattachment. Twelve die were attached to each flange, and each was thencooled to room temperature at approximately the same rate. Aftercooling, each flange was measured from center to edge for extent ofwarpage. The results are graphically illustrated in FIG. 11, withreference to the alphabetic designations, above, for each flange. As canbe seen, each of the frame-bearing stiffened flanges (B, D, and E) hadless warpage than the control flange A. Further, the flanges that hadboth frame and raised pedestal (D, E) had less warpage than those withonly a frame (B). Finally, the flange with frame and multiple pedestalsE had the best result (lowest warpage).

While at least one example embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexample embodiment or embodiments described herein are not intended tolimit the scope, applicability, or configuration of the invention in anyway. Rather, the foregoing detailed description will provide thoseskilled in the art with a convenient road map for implementing thedescribed embodiment or embodiments. It should be understood thatvarious changes can be made in the function and arrangement of elementswithout departing from the scope of the invention as set forth in theappended claims and the legal equivalents thereof.

1. A packaging flange comprising: a flange having a first surface and asecond surface opposed to the first, the first surface of the flangecomprising a first region having a lip extending thereon, the firstregion spaced from a second region adapted to receive at least one die,the lip configured to impart stiffness to the flange, the stiffnesssufficient to at least partially counteract thermally-induced stressesgenerated from coefficient of thermal expansion differentials betweenthe flange and a die during an elevated temperature die attachmentprocess.
 2. The flange of claim 1, wherein the second region comprises apedestal comprising a surface sized and shaped to support at least onedie thereon.
 3. The flange of claim 2, wherein the second regioncomprises a plurality of pedestals.
 4. The flange of claim 1, whereinthe lip extends around a peripheral region of the first surface of theflange to form a frame surrounding the second region.
 5. The flange ofclaim 4, wherein the second region comprises a pedestal comprising asurface sized and shaped to support at least one die thereon.
 6. Theflange of claim 4, wherein the second region comprises a plurality ofpedestals.
 7. The flange of claim 1, wherein the lip is molded in onepiece with the flange base.
 8. The flange of claim 1, wherein the ratioof stiffening lip height to flange thickness is from about 0.15 to about0.25.
 9. A die assembly comprising: a flange having a first surface anda second surface opposed to the first, the first surface of the flangecomprising a first region having a lip extending thereon, the firstregion spaced from a second region adapted to receive at least one die,the lip configured to impart stiffness to the flange, the stiffnesssufficient to at least partially counteract thermally-induced stressesgenerated from coefficient of thermal expansion differentials betweenthe substrate and a die to be mounted thereon in an elevated temperaturedie attachment process; at least one die mounted in the second region;and leads for electrical contact with the die, the leads configured topreserve a seating plane, the seating plane spaced from a plane of thesecond surface of the flange.
 10. The assembly of claim 9, wherein theleads are embedded in an organic composition and the molded leads haveend-portions embedded in the lip.
 11. The assembly of claim 9, whereinthe leads are embedded in an organic composition and the molded leadsextend from raised inboard end lead portions nearest the die to lowerout board lead end portions aligned substantially within the seatingplane.
 12. The assembly of claim 9, wherein the second region comprisesa pedestal comprising a surface adapted to support at least one diethereon.
 13. The assembly of claim 9, wherein the second regioncomprises a plurality of pedestals.
 14. A die assembly comprising: apackaging flange having a first surface and a second surface opposed tothe first, the first surface of the flange comprising a first regionhaving a lip extending thereon, the first region spaced from a secondregion adapted to receive at least one die, the flange having a bodyconfigured to impart stiffness to the flange, the stiffness sufficientto at least partially counteract thermally-induced stresses generatedfrom coefficient of thermal expansion differentials between the flangeand a die to be mounted thereon during an elevated temperature dieattachment process.
 15. The assembly of claim 14, wherein the body ofthe flange has thickness in the rage from about 65 to about 100 mils.16. The assembly of claim 14, wherein the first region extends along aside of the flange and the lip extends upward above the upper surface ofthe flange.
 17. The assembly of claim 16, further comprising leads, theleads having inboard end portions embedded along a length of a lip walland extending to an upper surface of the lip to permit attachment ofconnections to die on the flange.
 18. The assembly of claim 14 whereinthe first region extends along a side of the flange, and the flangecomprises a platform in the first region, the lip resting on theplatform.
 19. The assembly of claim 18 further comprising leads, theleads having inboard end portions embedded along a length of a lip walland extending to an upper surface of the lip to permit attachment ofconnections to die on the flange.
 20. The assembly of claim 14, furthercomprising at least one pedestal in the second region.